Radio frequency (RF) and microwave power amplifiers are used in communication systems to amplify RF and microwave modulated signals that carry information, respectively. In order to improve the utilization efficiency of a frequency spectrum, modulated signals of many standards, for example, the Code-Division Multiple Access (CDMA) and Orthogonal Frequency Division Multiplexing (OFDM) technologies, carry phase and amplitude information simultaneously. Information of these types of signals is modulated on both dimensions of amplitude and phase, so these types of signals are amplified by using a linear amplifier to ensure a small distortion generated in amplitude and phase. However, since it is inherently nonlinear, the power amplifier is bound to bring distortion of amplitude and phase to some extent. One method to address this problem is to enable the power amplifier to work in a quasi-linear state of a Class A or Class AB amplifier. By using an amplifier with large power to output small power (that is, the method of power back-off), the distortion of amplitude and phase caused by the power amplifier during the signal transmission is reduced. This method may ensure the linearity of the power amplifier, making it meet the requirements for the spectrum mask and the adjacent channel leakage ratio in the protocol. However, this method results in reduction in the efficiency of the power amplifier and increase in power consumption. If power consumption increases, bulky radiators and fans and other radiating devices will be needed, and thus product miniaturization will be difficult, cost of the system will increase, and reliability will be significantly reduced. Therefore, currently, various linearization technologies are proposed for eliminating nonlinearity of the power amplifier, so as to reduce the amount of back-off and improve the efficiency of the power amplifier. With the development of the digital signal processing technology, a solution applied widely now is as follows: using high-efficiency power amplifiers with a strong nonlinearity, such as a Class C, Class D, Class E, and Class F power amplifier, and applying the pre-distortion linearization technology where a nonlinear circuit is added in front of the power amplifier to compensate for the nonlinear distortion of the power amplifier, so as to improve the efficiency of the power amplifier as far as possible while the linear output is ensured. The advantages of the pre-distortion linearization technology lie in its stability, broader signal band, capability of processing signals that containing multi-carriers, and low cost.
Pre-distortion may be divided into two basic types: RF pre-distortion and digital pre-distortion (DPD). RF pre-distortion is commonly implemented by using an analog circuit, which has the advantages of simple circuit structure, low cost, and being easy to implement high frequency and broadband applications. However, the RF pre-distortion has the following disadvantages: limited improvement to the spectral regrowth component, and difficulty in eliminating high-order spectral components. Because of its low working frequency, the DPD may be implemented with a digital circuit, which is well adaptable. In addition, the high-order intermodulation distortion may be eliminated by increasing the sampling frequency and quantization order number in the DPD, so the DPD is quite a promising method. The DPD technology includes the table lookup method, polynomial method, neural network method and various other pre-distortion methods, in which the table lookup method is relatively simple, and is quite flexible in algorithm, making it a method used widely in the DPD technology.
The linear amplification of amplitude and phase of signals may be implemented by using a high-efficiency power amplifier with strong nonlinearity plus the DPD technology, for example, a Doherty power amplifier combined with the DPD, an Envelope Tracking (ET) power amplifier combined with the DPD, or an Envelope Elimination and Restoration (EER) power amplifier combined with the DPD. This kind of combination technology is commonly applied in the final power amplifier, that is, the final stage power amplifier to improve the efficiency of the final stage. While in the drive stage, a Class A or Class AB power amplifier for power back-off is applied, making it difficult to further improve the efficiency. If Doherty, ET, EER or other high-efficiency power amplifier technologies are used in the drive amplifier, the distortion of phase and amplitude of the drive amplifier and the distortion of phase and amplitude of the final power amplifier will result in a larger distortion amplitude, broader distortion bandwidth and increased memory effect of the final power amplifier's output, and difficulty in hardware design and DPD correction algorithm.